Preheat oven to 375°F (190°C). Grease a 9x13-inch baking dish.
In a medium saucepan, melt butter over medium heat. Whisk in the flour and cook for 1-2 minutes.
Gradually add milk and cream, whisking constantly until the mixture thickens. Add garlic powder, onion powder, paprika, salt, and pepper.
Remove from heat and stir in cheddar and mozzarella cheese until melted and smooth.
Layer half the potato slices in the prepared dish. Pour half the cheese sauce over the potatoes.
Repeat with remaining potatoes and cheese sauce.
Cover with foil and bake for 30 minutes. Remove foil and bake an additional 30-40 minutes until golden and bubbly.
Garnish with fresh parsley before serving, if desired.